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SubjectRe: [PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is any eligible zone
On Tue, Jul 05, 2016 at 11:38:06AM +0100, Mel Gorman wrote:
> On Tue, Jul 05, 2016 at 03:11:17PM +0900, Minchan Kim wrote:
> > > - if (i < 0)
> > > - goto out;
> > > + /*
> > > + * Only reclaim if there are no eligible zones. Check from
> > > + * high to low zone to avoid prematurely clearing pgdat
> > > + * congested state.
> >
> > I cannot understand "prematurely clearing pgdat congested state".
> > Could you add more words to clear it out?
> >
>
> It's surprisingly difficult to concisely explain. Is this any better?
>
> /*
> * Only reclaim if there are no eligible zones. Check from
> * high to low zone as allocations prefer higher zones.
> * Scanning from low to high zone would allow congestion to be
> * cleared during a very small window when a small low
> * zone was balanced even under extreme pressure when the
> * overall node may be congested.
> */

Surely, it's better. Thanks for the explaining.

I doubt we need such corner case logic at this moment and how it works well
without consistent scan from other callers of zone_balanced where scans
from low to high.

> > > + */
> > > + for (i = classzone_idx; i >= 0; i--) {
> > > + zone = pgdat->node_zones + i;
> > > + if (!populated_zone(zone))
> > > + continue;
> > > +
> > > + if (zone_balanced(zone, sc.order, classzone_idx))
> >
> > If buffer_head is over limit, old logic force to reclaim highmem but
> > this zone_balanced logic will prevent it.
> >
>
> The old logic was always busted on 64-bit because is_highmem would always
> be 0. The original intent appears to be that buffer_heads_over_limit
> would release the buffers when pages went inactive. There are a number

Yes but the difference is in old, it was handled both direct and background
reclaim once buffers_heads is over the limit but your change slightly
changs it so kswapd couldn't reclaim high zone if any eligible zone
is balanced. I don't know how big difference it can make but we saw
highmem buffer_head problems several times, IIRC. So, I just wanted
to notice it to you. whether it's handled or not, it's up to you.

> of things we treated inconsistently that get fixed up in the series and
> buffer_heads_over_limit is one of them.
>
> --
> Mel Gorman
> SUSE Labs
>
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